A virus is a very simple thing – a coat of protein wrapped around some genetic code (DNA or RNA). It’s not a cell and it’s not living.

Hijacking a host cell

A virus needs a host cell to be able to replicate itself. Once inside the host cell, it takes over. The cell is reprogrammed to produce the virus instead of doing what it was designed to do before. The virus is replicated thousands and thousands of times within that cell. Eventually, the cell bursts open and the multitude of viruses move around the body infecting other cells. This can happen within a few hours.


Mutation is when something changes as it is replicating. Viruses survive through mutation. This is a random act, rather than a deliberate act of survival.

Some of the ways that viruses replicate are quite sloppy compared to the cells in our bodies. Our cells have special enzymes that make sure that a new cell has copied the gene sequence perfectly, but a virus is not so exact. The host cell (under instruction from the genetic material of the virus) produces, say, 50,000 copies of the virus, but they are often not going to be 100% exact.

Dr Joanna Kirman from the Malaghan Institute of Medical Research in Wellington likens it to someone writing the alphabet. You might be writing the alphabet but instead of writing a J you make an extra N. It might be a small mistake, but a slight mutation such as this with a virus means our immune system might not recognise it. That means the mutated virus copy can infect a new cell making another 50,000 copies of the new mutation without our immune system trying to stop it. The mutation results in a new strain of virus.

You can have lots of strains (slight variations) of the same virus. Some viruses are sloppier than others when replicating and have many mutations. HIV (human immunodeficiency virus) is an example of this.

Reassortant strains

Influenza (flu) and rotavirus also produce many different strains. They do this in a slightly different way from mutation. These viruses can make reassortant strains, meaning the strain is produced from the genetic material from two or more similar viruses.

This happens when the gene material is chopped up in little pieces (rather than being in one long piece as with other viruses). Say you caught two different strains of the flu at once from two different people. A cell in your body could get infected with these two different strains. The strains can jumble up within the host cell and mix and match their little pieces of genetic material to create a new, dramatically different strain. This is how a flu pandemic occurs. The reassortant can spread quickly. Nobody’s immune system recognises the new virus strain so it is able to spread rapidly.

Scientists think that the swine flu occurred in the same way. A study of the gene sequence of the current circulating swine flu shows it’s a mix and match of swine flu, bird (avian) flu and human flu. It’s got genes from three different types of flu that have jumbled up together. Because it’s new, our immune systems don’t have any immunological memory of it. That’s why we are quite vulnerable to it.

Nature of science

Scientists study immune response to viruses so they can develop vaccines and therapies to stop them. However, scientific knowledge is based on our knowledge at the time observations are made. Because viruses mutate and re-assort rapidly, it is difficult for scientists to keep up with scientific knowledge of new viruses. As a result, pandemics can occur.

Useful links

What’s the threat of bird flu in New Zealand in winter? Dispel the myths, and get the facts from expert virologist Dr Sue Huang, in this audio from Te Papa.

    Published 8 November 2010, Updated 9 December 2014